Abstract

The Lusardi Formation, a previously undescnbed Cretaceous conglomerate, is discontinuously exposed over an area of 25 sq mi near Rancho Santa Fe, California. It unconformably overlies Late Jurassic to Early Cretaceous igneous and metamorphic rocks, and is unconformably overlain by the Eocene La Jolla Formation.

The Lusardi Formation is predominantly a reddish-brown boulder-to-cobble conglomerate with clasts that range up to 30 ft in diameter. Clast types include plutomc rocks, volcanic and volcanic-clastic rocks, and metasedimentary rocks derived from the underlying rock units. Thin lenses of well-sorted, medium-grained arkosic sandstone are locally present in the otherwise massive conglomerate.

The large size of the clasts, their local provenance, and color of the unit suggest terrestrial deposition, probably by fluvial and mass transport processes on alluvial fans. Many clasts are deeply weathered and friable, and the iron bearing minerals of the matrix are commonly altered to oxides, indicating lengthly subaerial weathering in a semi-arid climate after deposition.

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